You could be associated with someone's license which would give you full access to the forums.

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Just to spell this out, ask someone you've done work for to go to their Customer Areahttp://xenforo.com/customers/
login
click Forum Users on the right
add your forum name Clepto

Then you can access all the development forums and put your addons in here.

This has no effect on the customer's account. It gives you no extra access to do with their site, and no access to the customer area and their downloads of XF. You are not responsible for paying their licence.
It's ONLY for expanded forum access and use for you.

Except that such a thread or post would be off topic so would be removed.

The Resource Manager is now the only place where add-ons can be published here on XenForo.com.

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Yeah, I more meant that he can legally post it wherever he wants and link to it. Drop the URL in his sig, etc. Not that I'm suggesting it, I'm just saying that it's not illegal to write mods for xF and post them other than xenforo.com.